Elastic Grüneisen parameters in crystals of low symmetry

1638

TECHNICAL

Here the best fit with theory yields an equilibrium distribution coefficient of 0.13 for small amounts of tin in bismuth. With the viscosity value of liquid bismuth cited in[l], the diffusion coefficient of tin in liquid bismuth at about 275°C is calculated to be 1.6X 10m5cm*/sec. Niwa et af.[5] measured this diffusion coefficient in the temperature range 450”-600°C. The extrapolation of their data to 275°C results in a value of 2.8 X lo-” cm21sec. Acknon$edgements-Thanks are due to Dr. E. Bruninx and Mr. J. E. Crombeen for performing the radioactive tracer analysis. J. M. NOOTHOVEN H. M. G. J. TRUM

VAN GOOR

NOTES

in cubic crystals have been extensively investigated [ l-S]. Recently, Brugger [6] has formulated a generalized scheme of Gruneisen mode parameters in crystals of any symmetry. In the present note, the relations between the mode Gruneisen parameters and the pressure derivatives of the elastic moduli in several crystal classes of lower symmetry than cubic will be derived. The results will be applied to some materials where experimental data are available. An anisotropic continuum model, neglecting dispersion, will be assumed. In addition, the discussion will be limited to crystal structures where the linear thermal expansion axes is tensor (Y, referred to crystalline diagonal, and the elastic compliance moduli s,j obey the relation:

Philips Research Laboratories, N. V. Philips Gloeilampenfabrieken, Eindhoven, The Ne?her~and.s

SQ= 0:

REFERENCES 1. NOOTHOVEN

van

GOOR

J. M. and TRUM

H.M.G.J..J.Phys.Chem.Solids29.341

(1968). 2. SCHULTZ B. H. and NOOTHOVEN VAN GOOR J. M.. Philips Res. Rep. 19, 103 f 1964). 3. JAIN A. L. and KOENIG S. H., Phys. Rev. 127.443

(1961). 4. BURTON J. A., PRIM R. C. and SLIGHTER W. P.. J. them. Phys. 21.1987 (1953). 5. NIWA K., SHIMOJI M.. KADO s.,WATANABE Y. and YOKOKAWA T.. Trans. Am. Inst. Min. Metal& Engrs 209.96

J. Phys. Chem. Solids

( 1957).

Vol. 30. pp. 1638-1642.

(Received

I November

1968)

j=4,5,6.

(1)

These assumptions limit the treatment to hexagonal and orthocubic, tetragonal, rhombic crystals. The general mode Gruneisen parameter for a mode of wave vector q and polarization index p is defined as 173: yr;, = -[a In ap(q)/&rIjk]T; j. k = 1.2.3’

(2)

where q. are the Lagrangian strains, c+(q) the frequency of the mode q, p. Confining the strain to hysrostatic pressure P, we have the relation [6] : [a In ~,(q>/dPIT

Elastic Griineisen parameters in crystals of low ~ymmetry~

i= 1,2,3;

= --.&la

In

wp(qYhkL

(3)

where summation over repeated indices is implied. In our case, nij = 0 for i # j, and as a result of equation (1) we have: 3

THE GR~~NEISENparameters

play an important role in the theory of anharmonic effects in solids, and their properties and behavior *This work was performed under the auspices of the United States Atomic Energy Commission.

[dlnw,(q)l~Pl,=

Z: .dhP(q) i,j= 1

(4)

mode gamma tensor being diagonal. Now, as we neglect dispersion, w,(q) is given by:

the

w,(q) = qs,,(k 4).

(5)

TECHNICAL

1639

NOTES

(12)

But, 4 a (L,2 + L,z+ &2)--1’2

(6)

where L1, L2 and L3 are the lengths of the crystal parallel to the x, y and z, axes, and s,,(@,4) is the sound velocity for the mode q. p. Hence, one obtains the relation:

where p is the volume expansion thermal expansion coefficient (p = cyl+ CQ+ (Ye), Cv the specific heat at constant volume V, is given by 171

Y = [~C,(s)r”fs)]/[~C,(s)] [a In o,(q)/aPIT

= fZZK,*+ PP&~+

+ [a ]ns,,(k $)/W,

(7)

where KzT, KzT and KsT are the isothermal linear compressibilities in the directions of the crystalline axes, and l,m,n, are the direction cosines of q. Denoting the elastic stiffness modulus associated with the mode q, p by c,, one obtains: (a In w,(q)/W) = fZK,T+m”K2r+ n2KsT - 0.5 KI.T+ 0..5(8 In c,/iWT (8)

where KrT is the isothermal pressibility. Thus, we have

volume

$ s~~~j’(q) = 12K,T+m2K2T+n2K~3T i,j=I - 0.5K17T+ 0.5 (aln c,/P)

com-

T_

(9)

Defining now an averaged mode gamma

one obtains -f(q)

= [12K,T+m2K2T+n2K3T-O~5K~T

+0*5(a In

(13)

nZKzT)

~JJ~ITl/(ji,~zj).

(11)

As can be seen the individual mode gammas @(q) cannot be deduced from the pressure derivatives of the elastic moduli alone, but only their weighted average. In.order to determine the individual rip(q). uniaxial as well as hysrostatic pressure derivatives of the elastic moduli are required. The Griineisen parameter, defined by

where C,(q) is the specific heat associated with the mode q,p. The low and high temperature limits of the Griineisen parameter, yL and yH may be easily calculated, as at the low temperature limit C,(q) CC c~-~‘~, while in the high temperature limit C,(q) = kT. Hence, in these two limiting cases the sum in equation (13) may be evaluated in a straight forward manner. A computer program for the CDC 3600 computer which evaluates the averaged mode gammas yp(q) . as well as yt, and yH in crystals of cubic, hexagonal, tetragonal and orthorhombic symmetry has been written. The input data to the program are the room temperature elastic moduli, their pressure derivatives, and the low temperature elastic moduli. The program computes the sound velocity in any direction by calculating the eigenvalues of the Christoffel determinant[8], as well as the pressure derivatives in any direction. From the latter quantities the yp(q) as function of direction are determined. y,. and yH are evaluated by numerical quadrature.* The above program has been applied to three materials of hexagonal symmetry, where the values of the elastic moduli and their pressure derivatives are available, i.e. magnesium [9, lo], cadmium [ 11, 121 and cadmium sulfide [ 13,141. Since the hexagonal structure has transverse symmetry, the yp(q) need only be evaluated as a function of the latitude angle 8. The results of the computation are shown in Figs. l-3, where p = 1 is the longitudinal mode, p = 2 the fast shear mode, and *A write-up of the program author upon request.

may be obtained

from the

TECHNICAL

I 640

0

IO

20

30

NOTES

40

50

60

70

60

9 Fig. I. yp (q) as a function

0

IO

20

Fig. 2. y’(q)

30

40

as a function

of 0 for Mg.

50

60

of 0 for Cd.

70

00

90

TECHNICAL

1641

NOTES

I,0 P=l

0

-1.0

;i

a -2.0

-30

-40

L

0

IO

20

30

40

50

60

70

00

90

8 Fig. 3. Yp(q) as a function of 0 for CdS.

Table

1. The values of y,. and yH for Mg, Cd and CdS, as obtained from elastic and thermal Elastic Yl,

Mg Cd CdS

1.45 2.16 -2.19

expansion Thermal

data YH

1.52 2.06 -1.19

expansion

data YH

Yl.

1.40(lO”K) 2.10(20”K) -2.34(20”K)

data

[7] [16, 181 [IS. 181

1.50(300”K) 1.86(300”K) 0.45(300°K)

[7] [ 16,181 [15, 181

1642

TECHNICAL

p = 3 the slow shear mode. In Table I the values of yI. and yH as obtained from elastic data, are compared with the similar quantities derived from thermal expansion data. As can be seen, the agreement between the two sets of data is very good for magnesium and cadmium, while in the case of cadmium sulfide, there are large discrepancies between the thermal expansion and elastic yH- This is not surprising, as such discrepancies are expected when optical phonons contribute appreciably to the lattice vibration spectrum ]4], which is the case for cadmium sulfide. The thermal expansion value of y,, contains probably some error, as the values of the low temperature thermal expansion coefficient were determined from data of the lattice parameter as a function of temperature [ 151. It is interesting to note that the negative thermal expansion coefficient of cadmium at low temperatures[l& 171 is not reflected in yp(q) becoming negative. This is probably due to the dominance of the modes with positive values of yp(q) in the averaging process. On the other hand, in the case of cadmium sulfide, the ypyp(q) for the shear modes are both negative throughout, as well as yf,. This is in agreement with the fact that both thermal expansion coefficients of cadmium sulfide become negative at low temperatures. Acknowledgements

-The author wishes to express gratitude to E. S. Fisher for many helpful discussions comments. He is also indebted to the Metallurgy ision, Argonne National Laboratory, for the award summer appointment during the tenure of which work was done. Argonne National Laboratory, Argonne, U.S.A.

his and Divof a this

D. CERLICHt

I@. 60439,

REFERENCES 1. SLATER J. C., In introduction to Chemical Physics. McGraw Hill, New York (1939). 2. SHEARDF. W.,Phif. Mug. 3,138l (1958). tPermanent address: Physics University, Ramat Aviv, Israel.

Department,

Tel Aviv

NOTES 3. COLLlNS J. G., Phi/. Mug. 8,323 (1963). 4. SCHUELE D. E. and SMITH C. S., j. Phys. Chem. Solids 2580 1 (1964). A. 5. HlKI Y., THOMAS J. F.. and GRANATO V., Phys. Rev. 153,764 (1957). 6. BRUGGER K.. Phys. Rev. 137, A1826 (1965). 7. COLLINS J. G. and WHITE G. K.. In Progress m Lotis Temperature Physics (Edited by C. J. Garter). Vol. IV, p. 450. North-Holland, Amsterdam (1964). 8. MASON W. B., In Physical Acoustics and Properries ofSolids. Van Nostrand. Princeton. N.J. (1958). 9. EROS S. and SMITH C. S., Acta metall. 9, 14 (1961). 10. SCHMUNK R. E. and SMITH C. S., 1. Phys. Chem. Solids 9, 100 (1959).

If. GARLAND C. W. and SILVERMAN J., Phvs. Rev. 119, 1218 (1960); ibid. 127,2287 (1962). 12. CORLL J. A., Case Inst. Technol. O.N.R. Tech. Rep. No. 6 (1962). 13. GERLICH D.. J. Phys. Chem. Solids 28, 2575 (1967). 14. CORLLJ. A., Phys. Rev. 157,623 (2967). 15. REEBER R. R., Proc. Symp. Therm. Exp. Solids To be published. 16. MADAIAH N. and GRAHAM G. M., C’an.J. Phys. 42.22 1 ( 1964). 17. WHITE G. K., In Proc. 7th fnt. Conf. Low Temp. Phys.. p. 685. University ofToronto Press (1961). 18. Americun Institute of Physics Handbook. McGraw Hill, New York (1957).

1. Phys. Chem. Solids

Antiferromagnetic

(Received

28 August

vol. 30, pp. l642- 1644.

structures UBi*

of

1968; in revised form

USb

and

I October

1968)

URANIUM compounds with group VA elements (N, P, As, Sb, and Bi, denoted by v) that have the NaCl-type structure are antiferromagnetic. The values of the NeeI temperature (T,v), the paramagnetic Curie temperature (I?), and the paramagnetic moment (n,) increase along the series from UN to UBi. These properties and the high electrical conductivity were considered in a

THE

*This work was performed under the auspices of the United States Atomic Energy Commission.